Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 340

curing period, microbial cells obtained good nourishment, because the cement
mortar was still porous; but growth might not be proper due to the completely new
environment for microbes (Achal et al. 2011a , b , c ). It may also be possible that as
the pH of the cement remained high, cells were in inactive condition and as curing
period was increased, it started growing slowly. Upon cell growth, calcite would
have precipitated on the cell surface as well as within the cement mortar matrix.
Once many of the pores in the matrix were plugged, the flow of the nutrients and
oxygen to the bacterial cells stopped, eventually the cells either died or turned into
endospores and acted as an organic fiber, that may enhance the compressive
strength of the mortar cubes (Ramachandran et al. 2001 ). Despite the importance
of urease-producing bacteria in remediation of cracks and fissures toward
enhancement in the durability of building structures, very few bacteria have been
exploited. Bacteria inhabit all possible locations including extremes and exhibit
growth and reproduction in such environments. There is a need to explore extreme
alkaline environments to isolate indigenous bacteria that can survive in concrete
structures for effective biodeposition.
Recently, Achal et al. ( 2011a , b , c ) isolated bacteria from cement, Bacillus sp.
CT-5 and used to study compressive strength. They reported 36 % increase in
compressive strength of cement mortar with the addition of bacterial cells. Further,
it has suggested that due to the ability to tolerate high pH, Bacillus sp. CT-5
enhanced the compressive strength of cement mortar cubes significantly. To
determine whether the increase in compressive strength of the specimens prepared
with bacteria could be attributed to the microbial calcite deposition, biodeposition
was quantified by X-ray diffraction (XRD) analysis and visualized by SEM (De
Muynck et al. 2008a , b ; Achal et al. 2009 , 2011a , b , c )
From these findings, it can be concluded that compressive strength of cement
mortar increases with an addition of urease-producing microbes such as Bacillus,
Shewanella, and Arthrobacter species. This improvement in compressive strength
might be due to deposition on the microorganism cell surfaces and within the pores
of cement-sand matrix, which plug the pores within the mortar as a result of
biodeposition (Ramakrishnan et al. 1998). The strength of the bacteria modified
mortar increases due to the deposition of the new material gehlenite by the bac-
terial activity, which in turn increases mainly the uniformity of SiO 2 concentration
of the mortar. The high Ca/Si ratio at 10 5 cells/ml corresponds to the optimum
strength of bacteria modified mortar at such cell concentration. Also the protein
secreted by the bacterium leaches silica and helps in formation of new silicate
phases that fill the micropores. This protein increases the strength of mortar when
it is added separately (Ghosh et al. 2009 ). Pei et al. ( 2013 ) explored the role of
bacterial cell walls of Bacillus subtilis as a concrete admixture to improve the
mechanical performance of concrete. The bacterial cell walls are known to
mediate microbialy induced carbonate precipitation, a process in which CaCO3 is
formed from Ca2 + ions and dissolved CO2. Consistent with such knowledge,
incorporation of bacterial cell walls increased carbonation of Ca(OH)2 and for-
mation of CaCO3 in concrete. Furthermore, the bacterial cell walls significantly
increased